**A**s I am flying today to Seoul, for the Fall meeting of the Korean Statistical Society, a somewhat interesting paper in the New York Times about switching to alternatives for airline catering (if not air travel), starting with the figure that a passenger generates on average 1.5kg of waste per flight. And pointing out the conflicting issues in recycling food waste in most countries as they see it as imported waste and potential imported pathogens.and biohazards… While getting rids of plastic items is a tiny step in the right direction, especially because airlines do not sort between different kinds of garbage, a major step would be to avoid replacing them by another disposable item, especially heavier ones. From getting rid of providing food and drink (except water) on short and medium-haul flights to aim at healthy foods that do not require packaging or utensils. Like fruits. And asking passengers to carry their own garbage when leaving the plane could also enhance the realisation of the amount of garbage they thus produced. (On a recent early morning flight between Paris and Birmingham, the plane supposedly could not leave until the late delivery truck had brought croissants and drinks, as if passengers could not have abstained for the 55mn the flight lasted, especially when most of them were sleeping…) Nowadays. I usually travel with a water bottle that I fill before boarding after security and often skip meals on flights, but it invariably proves difficult to ask flight attendants to use my own reusable cup rather than a single-use plastic cup.

## Archive for recycling

## unbiased product of expectations

Posted in Books, Statistics, University life with tags ABC, g-and-k distributions, importance sampling, latent variable models, NP-complete problem, permanent of a matrix, pseudo-marginal MCMC, recycling, University of Warwick on August 5, 2019 by xi'an**W**hile I was not involved in any way, or even aware of this research, Anthony Lee, Simone Tiberi, and Giacomo Zanella have an incoming paper in Biometrika, and which was partly written while all three authors were at the University of Warwick. The purpose is to design an efficient manner to approximate the product of n unidimensional expectations (or integrals) all computed against the same reference density. Which is not a real constraint. A neat remark that motivates the method in the paper is that an improved estimator can be connected with the permanent of the n x N matrix A made of the values of the n functions computed at N different simulations from the reference density. And involves N!/ (N-n)! terms rather than N to the power n. Since it is NP-hard to compute, a manageable alternative uses random draws from constrained permutations that are reasonably easy to simulate. Especially since, given that the estimator recycles most of the particles, it requires a much smaller version of N. Essentially N=O(n) with this scenario, instead of O(n²) with the basic Monte Carlo solution, towards a similar variance.

This framework offers many applications in latent variable models, including pseudo-marginal MCMC, of course, but also for ABC since the ABC posterior based on getting each simulated observation close enough from the corresponding actual observation fits this pattern (albeit the dependence on the chosen ordering of the data is an issue that can make the example somewhat artificial).

## plastic oceans

Posted in Kids, pictures with tags ban plastic bags, cover, Jorge Gamboa, Mal de Ojo, National Geographic, oceans, plastic waste, pollution, recycling on May 28, 2018 by xi'an## conference carbon footprint

Posted in Kids, pictures, Running, Travel, University life, Wines with tags BNP11, conferences, Ecocup, fragile environment, ISBA 2018, plastic, recycling, Swarovski Optik Green Travel Award, tea cup, workshop on August 1, 2017 by xi'an**A**s a local organiser of the recent BNP 11 conference in Paris, and hence involved in setting and cleaning coffee breaks and [now famous] wine&cheese poster sessions, I was rather shocked by the amount of waste generated by those events, albeit aware of the importance of the social exchanges they induced… And thus got to wonder how the impact of those conference events could be reduced. One solution is the drastic one, namely to provide exactly nothing at all during the breaks between talks and expect anyone hungry or thirsty enough to bring one own’s food or drink. Another one, as suggested by my daughter at the dinner table, is to provide Ecocups, namely reusable plastic glasses that can given to all participants at the beginning of the conference. Or sold (or rented) to those who have not brought their own mug or bottle. (Of course, this may be a poor idea in that manufacturing and shipping a hard-plastic glass that most likely will be discarded after a few days may be more damaging than producing the equivalent number of “disposable” thin plastic glasses. And in the end all this agitation is peanuts compared with the impact of flying participants to the conference. For which I have no handy solution… As biking to the conference location is a privilege very few can enjoy.) Still, and even though this puts another stone in the already rocky organisers’ garden, I wish we could adopt more positive policies at the meetings we organise and sponsor.

## impressions from EcoSta2017 [guest post]

Posted in pictures, Statistics, Travel, University life with tags ABC convergence, Bayesian econometrics, Bayesian graphical model, copulas, Hong Kong, pseudo-marginal MCMC, quasi-Monte Carlo methods, recycling, SMC, stochastic volatility on July 6, 2017 by xi'an*[This is a guest post on the recent EcoSta2017 (Econometrics and Statistics) conference in Hong Kong, contributed by Chris Drovandi from QUT, Brisbane.]*

There were (at least) two sessions on Bayesian Computation at the recent EcoSta (Econometrics and Statistics) 2017 conference in Hong Kong. Below is my review of them. My overall impression of the conference is that there were lots of interesting talks, albeit a lot in financial time series, not my area. Even so I managed to pick up a few ideas/concepts that could be useful in my research. One criticism I had was that there were too many sessions in parallel, which made choosing quite difficult and some sessions very poorly attended. Another criticism of many participants I spoke to was that the location of the conference was relatively far from the city area.

In the first session (chaired by Robert Kohn), Minh-Ngoc Tran spoke about this paper on Bayesian estimation of high-dimensional Copula models with mixed discrete/continuous margins. Copula models with all continuous margins are relatively easy to deal with, but when the margins are discrete or mixed there are issues with computing the likelihood. The main idea of the paper is to re-write the intractable likelihood as an integral over a hypercube of ≤J dimensions (where J is the number of variables), which can then be estimated unbiasedly (with variance reduction by using randomised quasi-MC numbers). The paper develops advanced (correlated) pseudo-marginal and variational Bayes methods for inference.

In the following talk, Chris Carter spoke about different types of pseudo-marginal methods, particle marginal Metropolis-Hastings and particle Gibbs for state space models. Chris suggests that a combination of these methods into a single algorithm can further improve mixing. Continue reading